The present invention relates to wireless networks and, more specifically, to a method for addressing and minimizing interference due to overlapping wireless systems.
The 802.16h draft standard suggests two kinds of subframe structures which allocate for each system a period of time during which it is guaranteed interference free operation. During this interval, called the master system, no other system (slave system) is allowed to interfere. However, the other systems are allowed to operate in areas where they do not cause interference to the master system, thus improving the capacity of those slave systems, which would otherwise have to be totally silent.
FIG. 1 shows the interference due to overlapping systems when all the three systems have the same working channel. Three base stations S1, S2, and S3 are shown in FIG. 1. Single coverage areas include X corresponding to base station S1, Y corresponding to base station S2, and Z corresponding to base station S3. Dual coverage areas include A corresponding to base stations S1 and S2, B corresponding to base stations S1 and S3, and C corresponding to base stations S2 and S3. A triple coverage area D corresponds to all three base stations S1, S2, and S3.
FIG. 2 describes a first subframe structure of the community containing these three overlapping systems shown in FIG. 1. FIG. 2 shows a common subframe that is part of the MAC Frame when all the systems of a coexistence community may operate in parallel. The operation during these subframes may require limitations of the transmit power. FIG. 2 also shows a master subframe that is part of the MAC frame used by a specific system (master system) of a coexistence community to operate with reduced interference from its neighboring systems. FIG. 2 also shows a slave subframe that is part of the MAC frame coinciding with the master subframe in which all systems (other than the master) of the coexistence community have restricted operation.
FIG. 3 shows an alternative subframe structure of the community containing the three overlapping systems. The difference between FIGS. 2 and 3 is that in FIG. 2 each frame includes a master subframe of each cell in a community operating in the same working channel, whereas in FIG. 3 each frame includes a master subframe of only one cell in a community operating in the same working channel. This means that in FIG. 3 three frames are required to provide master subframes of the cells in a community with three cells operating in the same working channel. Both of the subframe structures of FIGS. 2 and 3 are known to those skilled in the art.
In a common subframe, the Base Station (“BS”) provides service to the Subscriber Station(s) (“SSs”) located in a non-overlapping area. In a master subframe, the BS has priority over the other systems which means it can service all the SSs associated with it. In a slave subframe, the BS should not interfere with the BS in a master state, which means it can provide service to the SSs in the non-overlapping area. FIG. 4 illustrates the regions being served during the various subframes when the subframe structure of FIG. 3 is used.
Note that in FIG. 4, Region X, which is served by base station S1, without any interference from base station S2 and S3, is served during all the subframes. An SS located in that region is served during the common frames as well as during the subframes in which S1 is either a slave or a master. Similarly, SSs located in regions Y and Z are also served by either S2 or S3 during all the subframes, as can be seen in FIG. 4.
On the other hand, S1 SSs located in region A suffer from interference from base station S2, consequently they are not served during the common subframes but only when S1 is the master. When S2 is the master, it serves all its SSs located in region A. When S3 is the master, area A can not be served by either S1 or S2 since no priority is allocated to either of them. Similarly, SSs in areas B and C should be served only during the master subframes of their corresponding system. In a common subframe, SSs in areas A, B, C, and D can not be served by any BS since no priority is allocated to any BS.
A slave hierarchy method has been proposed to give SSs in overlapping areas more chance to be served. The idea behind the method is to define a certain slave hierarchy. For example, after being the master a system becomes a “secondary master” and still has priority over all other systems in the neighborhood except the new master. Thus in the second subframe, base station S2 is the master, but base station S1 is now the “secondary master” and has priority over base station S3, in the third subframe, base station S3 becomes the master, while base station S2, the “secondary master,” has priority over base station S1. In the fourth subframe, again base station S1 is the master, while base station S3 is the second master.
Accordingly, a first new subframe structure is defined in FIG. 5 and a second new subframe structure is defined in FIG. 6. FIG. 5 shows a subframe structure with slave hierarchy including a common frame, master, secondary master, and slave for each of base stations S1, S2, and S3. FIG. 6 shows a subframe structure with slave hierarchy including a common frame/master, common frame/secondary master, and common frame/slave for each of base stations S1, S2, and S3. The difference between FIGS. 5 and 6 is that in FIG. 5 each frame includes a master subframe and secondary master of each cell in a community operating in the same working channel, whereas in FIG. 6 each frame includes a master subframe of only one cell and a secondary master subframe in a community operating in the same working channel. This means that in FIG. 6, three frames are required to provide master subframes of the cells and secondary master subframes in a community with three cells operating in the same working channel.
FIG. 7 shows the regions being served during the various subframes when the subframe structure of FIG. 6 with slave hierarchy is used.
As shown in FIG. 7, during the 3rd subframe base station S2 is the secondary master and has priority over base station S1, and the SSs in area A can be served. Similarly, S1 SSs in area B can be served during the second subframe and S3 SSs in area C can be served during the first subframe.
After this optimization, SSs in areas A, B, C, and D still can not be served by any BS since no priority is allocated to any BS in a common subframe. What is desired, therefore, is a further optimization to address this problem of the prior art.